The present invention relates generally to canceling high temperature leakage in resistors.
Modern electronic circuits, such as bandgap circuits and bias circuits, typically use resistors. Low-current circuits often require large-value resistors to provide low-level currents. Large-value resistors typically have correspondingly large areas, and also have high resistivities-per-square area. N-well-type resistors are a typical example of large-value resistors.
N-well resistors have undesirable leakage at high temperatures. The leakage is the result of the resistor body forming a back-biased junction with the surrounding well or substrate. At high temperatures, current leaks from the body of the resistor to the surrounding well or substrate. The body leakage becomes significant in typical applications as temperatures increase above 100 degrees Celsius. The leakage increases exponentially with temperature and is also proportional to the resistor die-area.
Low-current circuits, using large-value resistors and small bias currents, are particularly sensitive to high-temperature leakage. High-temperature leakage usually disrupts the desired operation of low-current circuits. Accordingly, the operation of low-current circuits at high temperatures is restricted such that many low-current circuits are limited to operation at temperatures of 120 degrees Celsius or lower.
Alternatively, resistors may be reduced in size, which reduces their leakage. However, reducing the size of the resistors requires larger bias currents in order to make the leakage in the reduced size resistors less significant. The disadvantage of using larger bias currents is that larger supply currents are required, which results in higher power consumption.
High-resistance poly resistors do not have significant leakage. High-resistance poly resistors can be used as an alternative to N-well resistors. However, high-resistance poly resistors may be sensitive to changes in value due to stress on the die that occurs during packaging. The changes in the resistor values limits the applications high-resistance poly resistors.
The present invention is directed to a circuit for canceling the effects of leakage in resistive circuits. A resistive circuit is used to generate a reference signal. The resistive circuit is subject to leakage at high temperatures. A matching leakage generator is selected to have leakage characteristics that are similar to the leakage characteristics of the resistive circuit. The matching leakage generator is used to generate a representative leakage current that is similar to the leakage current that is undesirably present in the resistive circuit. The representative leakage current is mirrored by a current mirror circuit. The mirrored leakage current is used to cancel the effects of leakage in the resistive circuit. Canceling the leakage current improves the accuracy of the reference signal at higher temperatures. The reference signal accuracy at higher temperatures is improved because the effects of leakage current (which are greater at higher temperatures) are cancelled by the mirrored leakage current.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detailed description of illustrated embodiments of the invention, and to the appended claims.